Trapping of low-refractive-index particles with azimuthally polarized beam

Azimuthally polarized beams, focused by a high-numerical-aperture (NA) objective lens, form a hollow intensity distribution near the focus, which is appropriate for trapping low-refractive-index particles, in contrast to common linearly polarized or radially polarized beams. In this paper, the field distribution of the azimuthally polarized beam focused by a high-NA objective is described by the vectorial diffraction integral, and then the radiation forces on spherical particles with different parameters such as radius and refractive index are calculated by the T-matrix method. Numerical results show that the azimuthally polarized beam not only can steadily trap low-refractive-index particles at the focus center but also can trap multiple high-refractive-index particles around the focus center by virtue of the hollow-ring configuration. The range of the sizes of low-refractive-index particles that can be trapped steadily are presented, corresponding to different parameters such as the NA of the objective and the relative refractive index, based on which the NA of the objective can be selected to trap the appropriate size of particles. (C) 2009 Optical Society of America

Brownian dynamics simulation of the crystallization dynamics of charged colloidal particles

Crystal formation process of charged colloidal particles is investigated using Brownian dynamics (BD) simulations. The particles are assumed to interact with the pair-additive repulsive Yukawa potential. The time evolution of crystallization process and the crystal structure during the simulation are characterized by means of the radial distribution functions (RDF) and mean square displacement (MSD). The simulations show that when the interaction is featured with long-range, particles can spontaneously assemble into body-centered-cubic (BCC) arrays at relatively low particle number density. When the interaction is short-ranged, with increasing the number density particles become trapped into a stagnant disordered configuration before the crystallization could be actualized. The simulations further show that as long as the trapped configurations are bypassed, the face-centered-cubic (FCC) structures can be achieved and are actually more stable than BCC structures. (C) 2010 Elsevier Inc. All rights reserved.

In-situ microscopic Fourier transform infrared spectroelectrochemistry of redox mechanism for electroactive species in polyelectrolyte

Studies for the development of the in-situ microscopic FTIR spectroelectrochemistry (MFTIRS) have been carried out in polyethylene glycol(PEG) polyelectrolyte, Redox reaction mechanisms of various electroactive substances involving inorganic salt, organic compound and inorganic polymeric particles have been studied.

The study of Ti doped ZSM-5 particles and cavities inside them

The TEM study of titanium-containing ZSM-5 zeolite before and after hydrothermal treatment was performed. The use of different TEM techniques, such as conventional TEM, HRTEM and EDX-line scans provides important information about the microscopic structure of the zeolite catalyst consisting from several phases. The hydrothermal treatment of zeolite powder leads to strong changes in the morphology of the constituting particles. They are characterized by a homogeneous structure before hydrothermal treatment while the occurrence of holes after thermal treatment was observed, These changes lead to the enrichment of zeolite with titanium which obviously enhance its catalytic activity. Some of the titanium surplus precipitates as TiO2 anatase nanoparticles within the holes. (C) 2001 Elsevier Science B.V. All rights reserved.

Spanning the scales of granular materials through microscopic force imaging.

If you walk on sand, it supports your weight. How do the disordered forces between particles in sand organize, to keep you from sinking? This simple question is surprisingly difficult to answer experimentally: measuring forces in three dimensions, between deeply buried grains, is challenging. Here we describe experiments in which we have succeeded in measuring forces inside a granular packing subject to controlled deformations. We connect the measured micro-scale forces to the macro-scale packing force response with an averaging, mean field calculation. This calculation explains how the combination of packing structure and contact deformations produce the observed nontrivial mechanical response of the packing, revealing a surprising microscopic particle deformation enhancement mechanism.

Characterization of virus-like particles associated with the human faecal and caecal microbiota

This work represents an investigation into the presence, abundance and diversity of virus-like particles (VLPs) associated with human faecal and caecal samples. Various methodologies for the recovery of VLPs from faeces were tested and optimized, including successful down-stream processing of such samples for the purpose of an in-depth electron microscopic analysis, pulsed-field gel electrophoresis and efficient DNA recovery. The applicability of the developed VLP characterization method beyond the use of faecal samples was then verified using samples obtained from human caecal fluid.

Modelling the rheology of anisotropic particles adsorbed on a two-dimensional fluid interface

We present a general approach based on nonequilibrium thermodynamics for bridging the gap between a well-defined microscopic model and the macroscopic rheology of particle-stabilised interfaces. Our approach is illustrated by starting with a microscopic model of hard ellipsoids confined to a planar surface, which is intended to simply represent a particle-stabilised fluid–fluid interface. More complex microscopic models can be readily handled using the methods outlined in this paper. From the aforementioned microscopic starting point, we obtain the macroscopic, constitutive equations using a combination of systematic coarse-graining, computer experiments and Hamiltonian dynamics. Exemplary numerical solutions of the constitutive equations are given for a variety of experimentally relevant flow situations to explore the rheological behaviour of our model. In particular, we calculate the shear and dilatational moduli of the interface over a wide range of surface coverages, ranging from the dilute isotropic regime, to the concentrated nematic regime.

Effective theory for trapped few-fermion systems

We apply the general principles of effective field theories to the construction of effective interactions suitable for few- and many-body calculations in a no-core shell model framework. We calculate the spectrum of systems with three and four two-component fermions in a harmonic trap. In the unitary limit, we find that three-particle results are within 10% of known semianalytical values even in small model spaces. The method is very general, and can be readily extended to other regimes, more particles, different species (e.g., protons and neutrons in nuclear physics), or more-component fermions (as well as bosons). As an illustration, we present calculations of the lowest-energy three-fermion states away from the unitary limit and find a possible inversion of parity in the ground state in the limit of trap size large compared to the scattering length. Furthermore, we investigate the lowest positive-parity states for four fermions, although we are limited by the dimensions we can currently handle in this case.

Recurrence of particles in static and time varying oval billiards

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Effect of anharmonicities in the critical number of trapped condensed atoms with an attractive two-body interaction

The quantitative effect in the maximum number of particles and other static observables was determined. A deviation in the harmonic trap potential that is effective only outside the central part of the potential, with the addition of a term that is proportional to a cubic or quartic power of the distance was considered. Results showed that this study could be easily transferred to other trap geometries to estimate anharmonic effects.

Scanning electron microscopic and profilometric study of different sharpening stones

Scaling and root planing contribute to the recovery of periodontal health. All periodontal instruments loose their fine cutting angle after use. To maintain this angle, correct sharpening is required using specifically designed stones. The characteristics of sharpening stones can be compared to the blade of the instruments and also transported to root surface during instrumentation. Root smoothness is related to the quality of the blade. Therefore, the purpose of this study was to evaluate the characteristics of 9 sharpening stones by scanning electron microscopic and profilometric analyses. Ceramic and Neumar stones were very fine and both may be recommended to maintain the sharpness of the instruments. Arkansas, Thompson and CE stones presented greater roughness with very regular and round particles, and are suitable for maintenance of the cutting angle. In addition, these stones may be indicated for the routine sharpening of the instruments that are partly dull. Oxide Aluminum, Carborundum and JON stones were the coarsest with large irregular particles and may be indicated for initial sharpening of totally dull instruments with completion of sharpening with finer stones.

Detection of isometric, dsRNA-containing viral particles in Colletotrichum gloeosporioides isolated from cashew tree

Fungi are disease-causing agents in plants and affect crops of economic importance. One control method is to induce resistance in the host by using biological control with hypovirulent phytopathogenic fungi. Here, we report the detection of a mycovirus in a strain of Colletotrichum gloeosporioides causing anthracnose of cashew tree. The strain C. gloeosporioides URM 4903 was isolated from a cashew tree (Anacardium occidentale) in Igarassu, PE, Brazil. After nucleic acid extraction and electrophoresis, the band corresponding to a possible double-stranded RNA (dsRNA) was purified by cellulose column chromatography. Nine extrachromosomal bands were obtained. Enzymatic digestion with DNAse I and Nuclease S1 had no effect on these bands, indicating their dsRNA nature. Transmission electron microscopic examination of extracts from this strain showed the presence of isometric particles (30-35 nm in diameter). These data strongly suggest the infection of this C. gloeosporioides strain by a dsRNA mycovirus. Once the hypovirulence of this strain is confirmed, the strain may be used for the biological control of cashew anthracnose.

Microscopic and spectroscopic analysis of biogenic aerosols

Aerosol particles are important actors in the Earth’s atmosphere and climate system. They scatter and absorb sunlight, serve as nuclei for water droplets and ice crystals in clouds and precipitation, and are a subject of concern for public health. Atmospheric aerosols originate from both natural and anthropogenic sources, and emissions resulting from human activities have the potential to influence the hydrological cycle and climate. An assessment of the extent and impacts of this human force requires a sound understanding of the natural aerosol background. This dissertation addresses the composition, properties, and atmospheric cycling of biogenic aerosol particles, which represent a major fraction of the natural aerosol burden. The main focal points are: (i) Studies of the autofluo-rescence of primary biological aerosol particles (PBAP) and its application in ambient measure-ments, and (ii) X-ray microscopic and spectroscopic investigations of biogenic secondary organic aerosols (SOA) from the Amazonian rainforest.rnAutofluorescence of biological material has received increasing attention in atmospheric science because it allows real-time monitoring of PBAP in ambient air, however it is associated with high uncertainty. This work aims at reducing the uncertainty through a comprehensive characterization of the autofluorescence properties of relevant biological materials. Fluorescence spectroscopy and microscopy were applied to analyze the fluorescence signatures of pure biological fluorophores, potential non-biological interferences, and various types of reference PBAP. Characteristic features and fingerprint patterns were found and provide support for the operation, interpretation, and further development of PBAP autofluorescence measurements. Online fluorescence detection and offline fluorescence microscopy were jointly applied in a comprehensive bioaerosol field measurement campaign that provided unprecedented insights into PBAP-linked biosphere-atmosphere interactions in a North-American semi-arid forest environment. Rain showers were found to trigger massive bursts of PBAP, including high concentrations of biological ice nucleators that may promote further precipitation and can be regarded as part of a bioprecipitation feedback cycle in the climate system. rnIn the pristine tropical rainforest air of the Amazon, most cloud and fog droplets form on bio-genic SOA particles, but the composition, morphology, mixing state and origin of these particles is hardly known. X-ray microscopy and spectroscopy (STXM-NEXAFS) revealed distinctly different types of secondary organic matter (carboxyl- vs. hydroxy-rich) with internal structures that indicate a strong influence of phase segregation, cloud and fog processing on SOA formation, and aging. In addition, nanometer-sized potassium-rich particles emitted by microorganisms and vegetation were found to act as seeds for the condensation of SOA. Thus, the influence of forest biota on the atmospheric abundance of cloud condensation nuclei appears to be more direct than previously assumed. Overall, the results of this dissertation suggest that biogenic aerosols, clouds and precipitation are indeed tightly coupled through a bioprecipitation cycle, and that advanced microscopic and spectroscopic techniques can provide detailed insights into these mechanisms.rn

Microscopic Picture of Aging in SiO2

We investigate the aging dynamics of amorphous SiO2 via molecular dynamics simulations of a quench from a high temperature Ti to a lower temperature Tf.We obtain a microscopic picture of aging dynamics by analyzing single particle trajectories, identifying jump events when a particle escapes the cage formed by its neighbors, and determining how these jumps depend on the waiting time tw, the time elapsed since the temperature quench to Tf. We find that the only tw-dependent microscopic quantity is the number of jumping particles per unit time, which decreases with age. Similar to previous studies for fragile glass formers, we show here for the strong glass former SiO2 that neither the distribution of jump lengths nor the distribution of times spent in the cage are tw dependent.We conclude that the microscopic aging dynamics is surprisingly similar for fragile and strong glass formers.

QUADRUPOLE LEVITATION OF PARTICLES IN A THERMODYNAMICALLY REALISTIC CLOUD ENVIRONMENT

Understanding clouds and their role in climate depends in part on our ability to understand how individual cloud particles respond to environmental conditions. Keeping this objective in mind, a quadrupole trap with thermodynamic control has been designed and constructed in order to create an environment conducive to studying clouds in the laboratory. The quadrupole trap allows a single cloud particle to be suspended for long times. The temperature and water vapor saturation ratio near the trapped particle is controlled by the flow of saturated air through a tube with a discontinuous wall temperature. The design has the unique aspect that the quadrupole electrodes are submerged in heat transfer fluid, completely isolated from the cylindrical levitation volume. This fluid is used in the thermodynamic system to cool the chamber to realistic cloud temperatures, and a heated section of the tube provides for the temperature discontinuity. Thus far, charged water droplets, ranging from about 30-70 microns in diameter have been levitated. In addition, the thermodynamic system has been shown to create the necessary thermal conditions that will create supersaturated conditions in subsequent experiments. These advances will help lead to the next generation of ice nucleation experiments, moving from hemispherical droplets on a substrate to a spherical droplet that is not in contact with any surface.